Of course not. One of the enduring debates is whether a target-date fund should aim to have its lowest amount of stocks on the date you retire, or at some point 10 to 20 years beyond your retirement date. Those who argue for a later date — including arch-rival Fidelity Investments — say that you can expect to be in retirement 20 or 30 years, and therefore should have .

Blackrock, which manages more than $500 billion in 401(k) assets, is a heavyweight in its field, and breaks with other big 401(k) providers by coming down on the “to” side — that is, of positioning the fund for maximum income at the retirement date, not ten to 20 years later.

The argument for hitting a fund’s lowest stock level at retirement is fairly simple: You won’t have a paycheck after you have your retirement party. If you retire on a particularly bad date — say, October 9, 2007 — you may not be able to recoup your losses if you have too large a stake in the stock market.

A fund’s glide path — that is, the rate at which it reduces its stock holdings — should remain stable after retirement, argues Blackrock. It’s a particularly bad idea to reduce your stock exposure after a big market downturn.

“Our analysis found that under any set of assumptions about investor risk preferences, capital markets or labor income, it is always optimal to have a flat post-retirement glide path,” said Matt O’Hara, Head of Research and Product Development for BlackRock’s US Retirement Group.